The present invention relates to conditioning sub-assemblies for conditioning a polishing pad (hereinafter referred to as "pad conditioning") that is employed in chemical-mechanical polishing (sometimes referred to as "CMP") of substrates. More particularly, the present invention relates to conditioning sub-assemblies including conditioning surfaces that have predetermined non-planar regions to effectively (1) maintain an existing polishing pad shape and/or (2) shape the polishing pad during pad conditioning.
As is well known in the art, an end effector and a conditioning disk are integral components in a conditioning sub-assembly that is typically employed during pad conditioning. Chemical-mechanical polishing (CMP) typically involves mounting a substrate, such as a semiconductor wafer, faced down on a holder and rotating the wafer face against a polishing pad mounted on a platen, which in turn rotates or orbits about an axis. A slurry containing a chemical that chemically interacts with the facing wafer layer and an abrasive that physically removes that layer is flowed between the wafer and the polishing pad or on the pad near the wafer. In semiconductor wafer fabrication, this technique is commonly applied to planarize various wafer layers such as dieletric layers, metallization layers, etc.
After polishing a significant number of wafers on the same polishing pad, the part of the polishing pad that contacts a center region of the wafer deteriorates to a greater extent than other regions of the polishing pad. This deterioration is attributed primarily to a constant down force applied by the wafer during CMP. As a result, well before the end of a production lot draws near, the wafers subjected to CMP experience a slower film removal rate at the center region of the wafer relative to the edge or peripheral regions of the wafer surface, which phenomenon is known in the art as "center slow polishing." "Production lot" refers to a collection of wafers that are fabricated as a group under substantially similar conditions and may ultimately be sold. Center slow polishing is undesirable because it leads to a non-uniformly polished wafer surface, i.e. the center region of the wafer surface is not polished to the same extent as the peripheral region of the wafer. This prematurely ends the life of the polishing pad. In a typical wafer fabrication facility, where several CMP apparatus are employed, the replacement cost of polishing pads can be significant.
One approach currently adopted to combat the non-uniformity produced by center slow polishing involves producing "center fast polishing" conditions by shaping the polishing pad appropriately. As used herein, "center fast polishing" refers to a situation where the wafer experiences a faster film removal rate at the center region of the wafer surface relative to the peripheral region of the wafer surface. In this approach, the polishing pad is initially shaped to produce center fast polishing within the limits of uniformity, i.e. the resulting wafer may have an overpolished center region, but the degree of center to edge non-uniformity is not unacceptably high. The center fast polishing conditions advanced at the beginning of the polishing pad life, therefore, effectively counteract the eventual center slow polishing conditions produced by the worn-out polishing pad.
Before a first wafer from a production lot is subjected to CMP, the polishing pad undergoes preconditioning (hereinafter referred to as "pad preconditioning") at which time the pad is broken-in typically by polishing dummy wafers on the polishing pad. FIG. 1A shows a cross-sectional view of a typical preconditioned polishing pad 10 shaped appropriately to produce center fast polishing conditions. The cross-sectional view of the polishing pad 10 shows two protruding domes 12, which are part of a circular protruding dome shaped wafer track, as shown in a top view of FIG. 1B. Polishing pad 10 includes a circular protruding dome shaped area 12 that is located between the center and peripheral regions of the polishing pad. FIG. 1B also shows a top view of a rotating preconditioned polishing pad 10 with a rotating wafer 16 that carves out a wafer track, which includes an inner boundary 18, an outer boundary 20 between which resides circular protruding dome shaped area 12 formed during pad preconditioning. Those skilled in the art will recognize that the width of the wafer track may be larger than the diameter of the wafer because during CMP, the rotating wafer also oscillates from side to side in a radial direction of the polishing pad. A wafer undergoing CMP on the protruding dome shaped wafer track has a larger polishing pad area available to polish the center region as opposed to the peripheral region of the wafer and will therefore experience center fast polishing. Furthermore, any down force applied on the wafer during CMP will have a greater impact at the center region of the wafer as it contacts the thickest portion of the protruding wafer track.
In order to achieve and maintain a high and stable polishing rate, the polishing pad undergoes conditioning on a regular basis, i.e. either every time after a wafer has been polished or simultaneously during wafer CMP. During pad conditioning, the polishing pad is abraded to form grooves, which facilitate slurry flow across the polishing pad and to the pad-wafer interface. FIG. 1B shows an end effector 22 of a representative conventional pad conditioning sub-assembly that contacts a polishing pad surface during pad conditioning. For simplifying illustration, end effector 22 and its movement from the center to edge on the polishing pad, during pad conditioning, are shown below polishing pad 10'. End effector 22 is typically cylindrically shaped and has two planar surfaces, one of which is designed to secure a conditioning disk (not shown to simplify illustration) that is also cylindrically shaped and has planar surfaces. The other planar surface of the conditioning disk is an abrasive surface that faces a polishing pad surface and abrades the pad surface during pad conditioning. The conventional pad conditioning sub-assembly, including end effector 22 and the conditioning disk, is attached to a pivoting conditioning arm (not shown to simplify illustration).
Before the conventional pad conditioning sub-assembly begins conditioning, end effector 22 is lowered automatically so that the abrasive surface of the conditioning disk may contact polishing pad 10, which may be rotating or orbiting. A pneumatic cylinder (not shown to simplify illustration) may then apply a downward force on end effector 22 such that the abrasive particles engage polishing pad 10 as the conditioning disk along with the end effector move on the polishing pad surface. For example, in the CMP apparatus such as Avanti 472, commercially available from Integrated Processing Equipment Corporation (IPEC) of Phoenix, Ariz. during pad conditioning, end effector 22 may slide along a length of the stationary conditioning arm in a radial direction from typically the center to the edge. As another example, in the CMP apparatus of Strasbaugh 6DS-JP, commercially available from Strasbaugh of San Luis Obispo, Calif. an end effector is attached to a conditioning arm, which moves in a radial direction of the polishing pad from an inner to an outer region of the polishing pad.
The software currently employed to automatically implement pad conditioning divides the distance from the edge to center of polishing pad 10 into various segments 26 of equal length, as shown in FIG. 1B. During pad conditioning, the conditioning disk moves from one segment to the next based on a conditioning recipe that is designed to maintain the shape of the polishing pad. The conditioning recipe assigns a predetermined "dwell time" and a predetermined polishing pad rotation rate for each segment. The term "dwell time" is well known in the art and refers to a period of time that the conditioning disk dwells or remains in contact with a particular area or segment of the polishing pad. In other words, during pad conditioning, the conditioning disk will contact a particular segment of the polishing pad for a predetermined duration, while the polishing pad rotates at a certain speed typically measured in rotations per minute (rpm). Conventional pad conditioning processes attempt to maintain the shape of the polishing pad by implementing an appropriate conditioning recipe, which may have varying dwell times and the polishing pad rotation rates from one segment to another. By way of example, in those segments where the protruding dome shaped area is to be maintained, the dwell time for the same polishing pad rotation rate may be shorter relative to other regions of the polishing pad that are substantially planar.
Unfortunately, the current pad conditioning process fails to effectively control or maintain the shape of the polishing pad to produce the desired center fast polishing conditions. By way of example, the segmented movement of the end effector in a radial direction of the polishing pad makes it very difficult to control the degree of protrusion of the dome shaped area on the polishing pad. As the end effector moves from its first position 22 at one segment to its second position 22' at a second segment, for example, it dwells on an overlap region 24 (shown as a shaded region in FIG. 1B based on two different dwell times, i.e. the conditioning disk contacts the overlapping region for a first dwell time associated with a first segment and then contacts the overlapping region again for a second dwell time associated with a second segment. Overlap region 24 is formed because the end effector, which typically has a diameter between about 4 and about 6 inches, dwells on segments that are typically shorter than the diameter of the end effector. Consequently, such overlap regions on the polishing pad make it difficult to control the degree of protrusion of the dome shaped wafer track. Furthermore, the segmented movement of the end effector is time consuming, which translates into a low throughput for the CMP process.
As another example, after pad life of a polishing pad from an initial pad lot has concluded, a new polishing pad may be employed from a subsequent pad lot, which may have a different hardness than the initial pad lot due to different processing conditions employed during polishing pad manufacturing. As a result, the appropriate conditioning recipe implemented in conditioning the old polishing pad from the initial pad lot may no longer be effective to maintain the shape of the new polishing pad. Those skilled in the art will recognize that under such circumstances, it may be necessary to determine a new pad conditioning recipe that effictively shapes the new polishing pad. This typically requires testing the polishing pad of the new pad lot and is therefore a time-consuming and an arduous task, which lowers wafer throughput.
What is therefore needed is an improved pad conditioning apparatus and process that effectively controls, maintains the polishing pad shape and is not time consuming.